The invention relates to a process for distilling crude polymethylol which is obtained in the preparation of polymethylols from alkanals and formaldehyde. The present invention further relates to a composition comprising polymethylol and 1 to 10 000 ppm by weight of an ester of polymethylol and of a hydroxy acid, and to the use thereof.
Polymethylols, for example neopentyl glycol (“NPG”) and trimethylolpropane (“TMP”), are used in the plastics sector for production of paint systems, coatings, polyurethanes and polyesters.
On the industrial scale, polymethylols are usually prepared by the Cannizzaro process. In order to prepare trimethylolpropane by this process, n-butyraldehyde is reacted with an excess of formaldehyde in the presence of an inorganic base. This likewise forms one equivalent of an inorganic formate as a coproduct. The separation of the salt of trimethylolpropane is complicated and requires additional work. Moreover, the inorganic salt—if it can be utilized in a profitable manner—must be worked up and purified. The occurrence of the coproduct otherwise constitutes a loss of the stoichiometrically used amounts of sodium hydroxide solution and formaldehyde. In addition, the yields in this inorganic Cannizzaro reaction are unsatisfactory in relation to n-butyraldehyde, since high-boiling constituents are formed in the course of the reaction, which cannot be utilized further.
Similar problems to those outlined for trimethylolpropane exist in the preparation of other polymethylols such as trimethylolethane (from n-propanal and formaldehyde) or trimethylolbutane (from n-pentanal and formaldehyde) or neopentyl glycol (from isobutyraldehyde and formaldehyde).
To avoid these disadvantages, WO 98/28253 disclosed a multistage process for preparing polymethylols, in which aldehydes having 2 to 24 carbon atoms are first condensed in a first stage (aldol reaction) with formaldehyde using tertiary amines as a catalyst to give the corresponding methylolalkanals, and then hydrogenated in a further stage (hydrogenation) to give the corresponding polymethylols. This multistage process is typically referred to as the hydrogenation process. This process is low in coproducts.
After the first stage of the hydrogenation process, unconverted aldehydes and a portion of the amine base are generally removed by distillation from the methylolalkanals formed and recycled.
In the distillation bottoms there remain—as well as the methylolalkanals formed—water, the adducts of formic acid and the tertiary amines used (amine formate) and formic acid itself.
In general, the polymethylolalkanal is obtained by these processes as a 20 to 70% by weight aqueous solution.
The polymethylolalkanal-containing solution is hydrogenated in a second stage in order to convert the polymethylolalkanals to the corresponding polymethylols, such as TMP or NPG.
The reaction discharge from the hydrogenation is typically an aqueous polymethylol mixture which comprises polymethylol, tertiary amine, water and organic secondary components, for example an adduct of tertiary amine and formic acid (amine formate). The aqueous polymethylol mixture is therefore typically purified by distillatively removing low boilers from the polymethylol compound.
The distillation discharge, which is discharged from the bottom of the evaporator or from the circulation system of the evaporator, comprises predominantly the polymethylol compound. According to the present description, this bottoms discharge is referred to as “crude polymethylol”.
In the context of this invention, it has been found that the crude polymethylol, as well as the polymethylol compound, comprises a significant amount of oxidation products of the dimethylolalkanals which have formed in the aldolization, such as hydroxypivalic acid (HPA) from hydroxypivalaldehyde.
It has also been found that these acidic compounds, in the course of distillation of crude polymethylol, can react in the column bottom with the polymethylol compounds to give esters. This generally leads to a yield loss of polymethylol compound. In addition, water is also released in the reaction. It has been found that, surprisingly, the water of reaction formed disrupts the condensation of the polymethylols, such as NPG.
A possible explanation for this is that, for example, NPG solidifies at approx. 139° C. and the top temperature is therefore not freely adjustable. A condensation in the case of NPG preparation is therefore generally performed at temperatures of more than 139° C. In this case, water behaves virtually inertly and entrains NPG out of the condenser, which further reduces the yield.